Ignition Sensitivity Analysis for Energy-Assisted Compression-Ignition Operation on Jet Fuels with Varying Cetane Number

Local deposition of thermal energy can be used to assist the combustion process of low cetane number (CN) fuels in compression-ignition engines, here termed energy-assisted compression ignition (EACI). In the current work, a commercial ceramic glow plug, operated beyond its conventional operation range, was used as the ignition assistant (IA) and sensitivity of fuel jet ignition to operation parameters was studied for two fuels using EACI in an optical engine. A design-of-experiments (DoE) study was devised to determine which engine parameters influenced the energy-assisted pilot injection ignition process the most. The DoE was constructed with four parameters: injection pressure, injected mass, injection timing, and ignition assistant temperature. The fuels used were F24 (Jet-A with military additives) with a cetane number of 48 and a cetane number 35 fuel mixture consisting of 60% F24 and 40% of an alcohol-to-jet fuel (ATJ), blended on a volumetric basis. Simultaneous OH chemiluminescence and schlieren imaging were employed along with in-cylinder pressure measurements to assess ignition delay, jet development, and the combustion process. A Gaussian Kriging emulator was utilized to perform the sensitivity analysis. The results indicate that ignition of the fuel jet directed toward the ignition assistant is mainly driven by the ignition assistant temperature, with around 90% of the sensitivity for both fuels ascribed to ignition assistant temperature. However, for ignition assistant surface temperatures greater than ~1350 K ignition delay is approximately constant with increasing temperature. Ignition delay from the time the fuel reaches the ignition assistant to start of combustion, here termed hot surface ignition delay, was analyzed to gain a better understanding of the EACI ignition process. The hot surface ignition delay for ignition assistant temperatures >1350 K ranged from approximately 200 to 500 μs, independent of fuel cetane number. This indicates that ignition for ignition assistant surface temperatures >1350 K is likely the result of high-temperature ignition chemistry.